In the manufacture of optical fiber, a glass preform rod is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and a glass fiber (also referred to as an optical fiber) is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower. Because the surface of the glass fiber is susceptible to damage caused by abrasion, it is necessary to coat the fiber after it is drawn but before it comes into contact with any surface. Inasmuch as the application of a coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must solidify before the glass fiber reaches the capstan. This is typically accomplished within a brief time interval by photocuring--a process in which the liquid coating material is converted to a solid upon exposure to electromagnetic radiation, preferably ultraviolet (UV) light.
Because the fibers are thin and flexible, they are readily bent when subjected to mechanical stresses such as those encountered during handling or exposure to varying temperature environments. Such bends in the fiber frequently result in optical loss that is much greater than the intrinsic loss of the fiber itself, and it has been found desirable to protect the glass fiber against such bending. Accordingly, the coating material is required to cushion the glass fiber against bends and two layers of coating materials are typically applied to the drawn optical fiber. An inner (primary) coating, having a relatively low equilibrium modulus, is applied directly to the glass fiber; and an outer (secondary) coating, having a relatively high equilibrium modulus, surrounds the primary coating. Together, these coatings protect the inherently high tensile strength of the glass fiber as long as the primary coating remains bonded to the glass. Moreover, it is desirable for the primary coating to adhere to the glass fiber when pressure is applied. Adhesion is measured by a pull-out test that measures the force required to remove a 1 centimeter length of coating material from a coated glass fiber by pulling it along the longitudinal axis of the fiber. Low adhesion is a problem that, in the worst case, leads to "delamination" and the ensuing incursion of water, particularly upon exposure to high humidity, which attacks the glass surface and reduces tensile strength.
In what appears to be a contradictory requirement, it is also desirable to be able to easily strip the primary coating from the glass fiber--particularly when a number of fibers are bonded together in an array such as shown in U.S. Pat. No. 4,900,126. Such an array is frequently referred to as a "ribbon." Indeed, if the coating materials cannot be cleanly and easily stripped, then splicing and connectorizing operations will be seriously hampered.
U.S. Pat. No. 5,373,578 discloses a strippable coating material for an optical fiber that achieves improved strippability by the addition of a non-crosslinked hydrocarbon component as an additive. This particular coating material, however, exhibits a pull-out force that exceeds 3.0 pounds per centimeter of length (lb/cm) after 90 days aging, which is higher than desired.
U.S. Pat. No. 4,962,992 discloses desirable properties in an optical fiber coating material, and states that delamination may be prevented by controlling the adhesion level to be in the range of about 1 to 5 lb/cm. Nevertheless, it is desirable to have a low adhesion level without being susceptible to delamination.
What is needed is a coating material for an optical glass fiber that leaves a low residue on the glass after stripping, particularly from an array of glass fibers, while maintaining sufficient adhesion to the fibers so that delamination is avoided.